Get the online manual and verify the RAM positions. Often the RAM doesn't go next to each other.
If you have 4 slots, the RAM could go 1 and 2 or it could go 1 and 3. You might have put it back into the wrong slots.
If it still doesn't work, disconnect EVERYTHING. That includes power to the hard drive/CD/etc... and the IDE/SATA cables also. You can take out the Video card, sound card, lan card or anything else you have.
Try booting with a bare naked mother board. See what type of code you get then. If the problem still exists, then it could be a fan or power problem, or even a motherboard itself. If you get a different code then you know there is something wrong with the config or equipment you disconnected. Reconnect one thing at a time, starting with cards.
Since you cleaned it up, make sure the fans are plugged into the right headers on the motherboard. Some BIOS looks for certain RPMs of the fans before it will allow booting. If you accidentally put your CPU fan into a normal fan header, then the BIOS thinks your CPU fan is not spinning at all, and it won't allow boot.
Just FYI, most system failures are power supply. If you have a spare laying around you can plug into the MB and try to boot up it would help a lot.